Red Giants and Neutron Stars and Gravitational Waves, Oh My!

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at

Title: Prospects for Multimessenger Observations of Thorne-Żytkow Objects
Authors: Lindsay DeMarchi, J. R. Sanders, and Emily M. Levesque
First Author’s Institution: Northwestern University
Status: Published in ApJ

The universe is full of different types of stars, including big ones called red giants. But what if some of those red giants are hiding another star inside them?

Two Stars for the Price of One?

illustration of a large red star surrounded by spherical shells of mass

Artist’s illustration of a red giant star expelling mass at the end of its life. [JAXA]

A Thorne-Żytkow object (TZO) is a very special type of hybrid object that consists of two stars: a red giant (or supergiant) and a neutron star that lies at the core of the red giant. One way a TZO could be created is from the evolution of a close binary of two massive stars (> 8 solar masses) orbiting each other. Once the more massive star from the pair reaches the end of its lifetime, it will go supernova and leave behind a small, dense neutron star. This process could cause the neutron star and the remaining massive star to inspiral, allowing the red giant to swallow the tiny, but dense, neutron star — perhaps the most epic fit of celestial sibling jealousy!

The Challenges of Detecting a TZO

Though TZOs were first proposed in 1977, they remain extremely hard to detect and have never been observationally confirmed to exist. One of the issues is that a TZO doesn’t look that different from a red giant. Due to the presence of the neutron star core, however, TZOs should have different chemical abundances than red giants. Using this clue, one of the authors of today’s paper, Dr. Emily Levesque, identified a strong TZO candidate in the Small Magellanic Cloud in 2014 (read a bite about it here!). This star (known as HV 2112) has the chemical composition expected for TZOs — though it still may simply be a weird red giant without a neutron star core.

Besides TZOs being difficult to “visually” distinguish from red giants, they can also be difficult to gravitationally distinguish from standalone neutron stars. While it’s forming, a TZO will emit gravitational waves (GWs) at ~10 Hz frequencies that ground-based detectors like LIGO can’t see due to seismic noise coming from the Earth. After formation, a TZO will emit gravitational waves from its neutron core “spinning down” (spinning slower and slower). But spinning down is what neutron stars living outside of TZOs are also doing (we can see this happen with pulsars, for example), making it hard to tell TZOs and standalone neutron stars apart using just gravitational waves.

Where Does One Find a TZO?

The good news is that gravitational waves and visual identification of red giants can be used in unison to better identify TZOs! To that end, the authors of today’s paper identified a few nearby red (super)giant-rich regions that could be good candidates for hosting TZOs. They settled on one group of red supergiants in a region of the sky called the Scutum–Crux arm. The region is named RSGC1 and is about 6.6 kpc away from Earth. It is also very compact, about 10 million years old, and has around 210 massive stars. Its distance and small size make it ideal to scan for gravitational signatures, while its age and massive star population mean TZOs would have had time and the opportunity to form.

The authors carefully modeled what the gravitational signature of a TZO located in RSGC1 would look like (see the figure below). They took into account the properties of the red giant cluster, such as its distance and size. They also considered how fast neutron stars tend to spin down, which depends on their spin frequency to some power n, where 2 < n < 7. The authors consider a range of options for n that correspond to three different models for how the neutron star at the center of the TZO would spin down. Finally, they use what is known as the spindown limit, meaning that they assume all the energy from the slowing of the neutron star’s rotation is released as GWs. In reality, some of this energy could be used elsewhere — meaning that their calculation below is an upper limit for GW signals of TZOs in RSGC1.

Plot of strain vs. frequency.

Plot of strain — the strength of gravitational waves that LIGO is sensitive to — vs. frequency range of the LIGO detector. The curved black line shows the noise curve of the LIGO detector: LIGO can detect everything above the curve. The authors also show their calculations for GW signatures of TZOs in RSGC1 given three different models for neutron star spin as horizontal lines, shown in red (n=2), blue (n=5), and gray (n=7). All three lines are well above the LIGO sensitivity curve at frequencies greater than about 20 Hz, meaning that LIGO could indeed help detect potential TZOs in RSGC1! [DeMarchi et al. 2021]

A New Tool for Finding TZOs

The authors have shown that the expected gravitational signatures for TZOs in RSGC1 are well above the noise threshold of LIGO, meaning that any neutron star cores would likely be detectable! The next step is to look for such signatures in archival LIGO data and compare them with observational data. If astronomers can find both a gravitational wave signature of a neutron star and a visual signature of a red giant emanating from the same source, it will be the strongest evidence yet of a TZO: a star within a star!

Original astrobite edited by James Negus.

About the author, Luna Zagorac:

I am a PhD candidate in the Physics Department at Yale University. My research focus is ultra light (or fuzzy) dark matter in simulations and observations. I’m also a Franke Fellow in the Natural Sciences & Humanities at Yale working on a project on Egyptian archaeoastronomy, another passion of mine. When I’m not writing code or deciphering glyphs, I can usually be found reading, doodling, or drinking coffee.